The fidelity of DNA replication, recombination, and repair is essential for maintaining genome stability, and all of these processes depend on 5′→3′ exonuclease enzymes which are present in all organisms. For DNA repair, these enzymes are required for damaged fragment excision and recombinational mismatch correction. For replication, these nucleases are critical for the efficient processing of Okazaki fragments during lagging strand DNA synthesis. In Escherichia coli, this latter activity is provided by DNA polymerase I (PolI); E. coli strains with inactivating mutations in the PolI 5′→3′ exonuclease domain are not viable due to an inability to process Okazaki fragments. Eukaryotic DNA polymerases, however, lack an intrinsic 5′→3′ exonuclease domain, and this critical activity is provided by the multifunctional, structure-specific metallonuclease FEN-1 (five′ exonuclease-1 or flap endonuclease-1), which also acts as an endonuclease for 5′ DNA flaps (Reviewed in Hosfield et al., 1998a, Cell, 95:135).
Methods of detecting and/or measuring a nucleic acid wherein an enzyme produces a labeled nucleic acid fragment are known in the art.
U.S. Pat. Nos. 5,843,669, 5,719,028, 5,837,450, 5,846,717 and 5,888,780 disclose a method of cleaving a target DNA molecule by incubating a 5′ labeled target DNA with a DNA polymerase isolated from Thermus aquaticus (Taq polymerase) and a partially complementary oligonucleotide capable of hybridizing to sequences at the desired point of cleavage. The partially complementary oligonucleotide directs the Taq polymerase to the target DNA through formation of a substrate structure containing a duplex with a 3′ extension opposite the desired site of cleavage wherein the non-complementary region of the oligonucleotide provides a 3′ arm and the unannealed 5′ region of the substrate molecule provides a 5′ arm. The partially complementary oligonucleotide includes a 3′ nucleotide extension capable of forming a short hairpin either when unhybridized or when hybridized to a target sequence at the desired point of cleavage. The release of labeled fragment is detected following cleavage by Taq polymerase.
U.S. Pat. Nos. 5,843,669, 5,719,028, 5,837,450, 5,846,717 and 5,888,780 disclose the generation of mutant, thermostable DNA polymerases that have very little or no detectable synthetic activity, and wild type thermostable nuclease activity. The mutant polymerases are said to be useful because they lack 5′ to 3′ synthetic activity; thus synthetic activity is an undesirable side reaction in combination with a DNA cleavage step in a detection assay.
U.S. Pat. Nos. 5,841,669, 5,719,028, 5,837,450, 5,846,717 and 5,888,780 disclose that wild type Taq polymerase or mutant Taq polymerases that lack synthetic activity can release a labeled fragment by cleaving a 5′ end labeled hairpin structure formed by heat denaturation followed by cooling, in the presence of a primer that binds to the 3′ arm of the hairpin structure. Further, U.S. Pat. Nos. 5,843,669, 5,719,028, 5,837,450, 5,846,717 and 5,888,780 teach that the mutant Taq polymerases lacking synthetic activity can also cleave this hairpin structure in the absence of a primer that binds to the 3′ arm of the hairpin structure.
U.S. Pat. Nos. 5,843,669, 5,719,028, 5,837,450, 5,846,717 and 5,888,780 also disclose that cleavage of this hairpin structure in the presence of a primer that binds to the 3′ arm of the hairpin structure by mutant Taq polymerases lacking synthetic activity yields a single species of labeled cleaved product, while wild type Taq polymerase produces multiple cleavage products and converts the hairpin structure to a double stranded form in the presence of dNTPs, due to the high level of synthetic activity of the wild type Taq enzyme.
U.S. Pat. Nos. 5,843,669, 5,719,028, 5,837,450, 5,846,717 and 5,888,780 also disclose that mutant Taq polymerases exhibiting reduced synthetic activity, but not wild type Taq polymerase, can release a single labeled fragment by cleaving a linear nucleic acid substrate comprising a 5′ end labeled target nucleic acid and a complementary oligonucleotide wherein the complementary oligonucleotide hybridizes to a portion of the target nucleic acid such that 5′ and 3′ regions of the target nucleic acid are not annealed to the oligonucleotide and remain single stranded.
U.S. Pat. Nos. 5,846,717, 6,090,543, 6,001,567, 6,090,606, 5,985,557 and 5,994,069 relate to methods for forming a nucleic acid cleavage structure on a target sequence and cleaving the nucleic acid cleavage structure in a site specific manner. These patents also relate to using the 5′ nuclease activity of a variety of enzymes to cleave the target-dependent cleavage structure, thereby indicating the presence of a specific nucleic acid sequence or variations thereof.
Methods of detecting and/or measuring a nucleic acid wherein a FEN-1 enzyme is used to generate a labeled nucleic acid fragment are known in the art.
U.S. Pat. No. 5,843,669 discloses a method of detecting polymorphisms by cleavase fragment length polymorphism analysis using a thermostable FEN-1 nuclease in the presence or absence of a mutant Taq polymerase exhibiting reduced synthetic activity. According to this method, double stranded Hepatitis C virus (HCV) DNA fragments are labeled by using 5′ end labeled primers (labeled with TMR fluorescent dye) in a PCR reaction. The TMR labeled PCR products are denatured by heating to 95° C. and cooled to 55° C. to generate a cleavage structure. U.S. Pat. No. 5,843,669 discloses that a cleavage structure comprises a region of a single stranded nucleic acid substrate containing secondary structure. Cleavage is carried out in the presence of CleavaseBN nuclease, FEN-1 nuclease derived from the archaebacteria Methanococcus jannaschii or both enzymes. Labeled reaction products are visualized by gel electrophoresis followed by fluoroimaging. U.S. Pat. No. 5,843,669 discloses that CleavaseBN nuclease and Methanococcus jannaschii FEN-1 nuclease produce cleavage patterns that are easily distinguished from each other, and that the cleavage patterns from a reaction containing both enzymes include elements of the patterns produced by cleavage with each individual enzyme but are not merely a composite of the cleavage patterns produced by each individual enzyme. This indicates that some of the fragments that are not cleaved by one enzyme (and which appear as a band in that enzyme's pattern) can be cleaved by a second enzyme in the same reaction mixture.
Lyamichev et al. disclose a method for detecting DNAs wherein overlapping pairs of oligonucleotide probes that are partially complementary to a region of target DNA are mixed with the target DNA to form a 5′ flap region, and wherein cleavage of the labeled downstream probe by a thermostable FEN-1 nuclease produces a labeled cleavage product. Lyamichev et al. also disclose reaction conditions wherein multiple copies of the downstream oligonucleotide probe can be cleaved for a single target sequence in the absence of temperature cycling, so as to amplify the cleavage signal and allow quantitative detection of target DNA at sub-attomole levels (Lyamichev et al., 1999, Nat. Biotechnol., 17:292).
The polymerase chain reaction (PCR) technique, is disclosed in U.S. Pat. Nos. 4,683,202, 4,683,195 and 4,800,159. In its simplest form, PCR is an in vitro method for the enzymatic synthesis of specific DNA sequences, using two oligonucleotide primers that hybridize to opposite strands and flank the region of interest in the target DNA. A repetitive series of reaction steps involving template denaturation, primer annealing and the extension of the annealed primers by DNA polymerase results in the exponential accumulation of a specific fragment whose termini are defined by the 5′ ends of the primers. PCR is reported to be capable of producing a selective enrichment of a specific DNA sequence by a factor of 109. The PCR method is also described in Saiki et al., 1985, Science, 230:1350.
While the PCR technique is an extremely powerful method for amplifying nucleic acid sequences, the detection of the amplified material requires additional manipulation and subsequent handling of the PCR products to determine whether the target DNA is present. It is desirable to decrease the number of subsequent handling steps currently required for the detection of amplified material. An assay system, wherein a signal is generated while the target sequence is amplified, requires fewer handling steps for the detection of amplified material, as compared to a PCR method that does not generate a signal during the amplification step.
U.S. Pat. Nos. 5,210,015 and 5,487,972 disclose a PCR based assay for releasing labeled probe comprising generating a signal during the amplification step of a PCR reaction in the presence of a nucleic acid to be amplified, Taq polymerase that has 5′ to 3′ exonuclease activity and a 5′, 3′ or 5′ and 3′ end-labeled probe comprising a region complementary to the amplified region and an additional non-complementary 5′ tail region. U.S. Pat. Nos. 5,210,015 and 5,487,972 disclose further that this PCR based assay can liberate the 5′ labeled end of a hybridized probe when the Taq polymerase is positioned near the labeled probe by an upstream probe in a polymerization independent manner, e.g. in the absence of dNTPs.
There is a need in the art for a method of generating a signal that can be easily distinguished from oligonucleotide fragments that may arise from nuclease contaminants, using a nucleic acid cleavage reaction.
There is also a need in the art for a method of generating a signal wherein a target is amplified in either a linear or an exponential manner.
There is a need in the art for a method of generating a signal wherein a target is amplified in either a linear or an exponential manner in an isothermal reaction that utilizes a probe comprising secondary structure wherein some or all of the self-complementary regions of the probe that anneal to form the secondary structure are melted when the probe hybridizes with a target nucleic acid, thereby reducing non-specific binding of the probe to the target, and increasing the specificity of the assay.
U.S. Pat. Nos. 5,843,669, 5,719,028, 5,837,450, 5,846,717 and 5,888,780 also disclose a method of cleaving a labeled nucleic acid substrate at naturally occurring areas of secondary structure. According to this method, biotin labeled DNA substrates are prepared by PCR, mixed with wild type Taq polymerase or CleavaseBN (a mutant Taq polymerase with reduced synthetic activity and wild type 5′ to 3′ nuclease activity), incubated at 95° C. for 5 seconds to denature the substrate and then quickly cooled to 65° C. to allow the DNA to assume its unique secondary structure by allowing the formation of intra-strand hydrogen bonds between the complementary bases. The reaction mixture is incubated at 65° C. to allow cleavage to occur and biotinylated cleavage products are detected.
There is a need in the art for a method of generating a signal using a nucleic acid cleavage reaction wherein the cleavage structure is not required to contain areas of secondary structure and wherein a target is amplified in either a linear or an exponential manner.